Knee replacement surgery, much like other major surgeries carries a risk for certain complications, a few of which are life-threatening and some can lead to an unsuccessful or unsatisfactory outcome. Regarding the time horizon, some post knee replacement complications can manifest within days or months while some other problems will surface only after a few years.
Although the knee replacement procedure has become relatively safer with negligible mortality risks,all possible knee replacement complications should be carefully evaluated by you and your doctor. The risks of various complications vary from patient to patient by several factors such as age, weight, prior illnesses such as diabetes, cardiac issues, bone density issues. Therefore, only analysis that is based on all available medical facts about a patient can indicate the risks that apply strongly to a patient. Effective steps can then be taken to either mitigate those or carefully evaluate the pros and cons of the knee replacement surgery.
As with any surgery, knee replacement surgery or knee arthroplasty has risks as well as benefits. While most people who have a knee replacement surgery do not experience serious complications, still just like any other major surgery, a knee replacement procedure carries both life-threatening risks and other complications.
Total knee replacement (TKR) has become an acceptable method of treating chronic arthritis of the knee joint, at a stage when all other alternative treatments have failed to provide any relief. It is a procedure that is typically reserved when the patient’s quality of life has significantly deteriorated due to the chronic pain and disability. The operative procedure must be performed with precise skill and accuracy.Careful alignment of the prosthetic components is critical for minimizing complications. Selection of the right technology for implants is also of vital importance.
Most knee replacement operations are problem-free, but about one person in every 20,i.e., 5 percent may have some post knee replacement problems. Most of these complications are minor and can be successfully treated. The severity of complications depends on several factors including patient’s age, general health, excess weight or obesity, and the presence of any other illness such as diabetes, cardiac issues. 
According to the NIH (National Institutes of Health), post-surgical knee replacement complications ratio is very less compared to the success rates after knee replacement; but it is important that you consider your risk and discuss with your doctor. Serious complications, such as knee infection, occur in approximately 2.0% of patient . Major complications like heart attack or stroke occur even less frequently. (Of course, chronic diseases can increase the possibility of complications). 
The post-surgery complications also vary as per the patient’s health condition. For example, an overweight 65 years old who has led a mostly sedentary lifestyle and had a family history of osteoporosis is likely to have a higher risk of complications after surgery than an individual with strong bones and a more active lifestyle. Genetics, lifestyle, and family history play a significant part in the success rate of any surgery.
Conditions that can cause a higher than average rate of post knee replacement complications: 
- Inflammatory arthritis, such as rheumatoid
- History of venous thrombo-embolism (a blood clot that forms within a vein)
- History of pulmonary embolism (blockage in the main artery of the lung)
- History of Osteoporosis
- Physical inactivity
- Excessive alcohol consumption
- Prior injuries or surgeries
Types of knee replacement risks
Seriousknee replacement complications are reported in 0.8%-1.9 % of TKRs. The AAOS (American Academy of Orthopedic Surgeons) published a guideline and evidence report  on risks and complications after knee replacement surgery such as:
- Joint Infection, which occurs in <1% of patients
- DVT (Deep vein thrombosis)seen in up to at least 15% of patients
- Nerve injuries occur in 1–2% of patients
- Persistent pain or stiffness occurs in 8–23% of patients
- Prosthesis failure occurs in approximately 2% of patients at five years
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Most common knee replacement complications
Is knee surgery safe? This is the ultimate question every patient has, but more effort is needed to understand all the risks that apply to one’s particular health condition. Educating oneself through the internet is a goodapproach, but it is best that the patient seeks a reliable and sound second opinion from independent experts on a matter as critical as undergoing major surgery.
Post-surgery formation of blood clots in the deeper veins is called Deep Vein Thrombosis (DVT) and is a rather benign treatable condition. Several research studies report that the formation of such clots is quite common. Among studies carried out on Western patients, it was found in the range of 46% to 84% patient without chemical prophylaxis [3,4,5]. Research conducted on Asian population shows a lower rate of DVT hovering between 14%. DVT by itself is not a fatal complication and is usually rectified by anticoagulant drugs, usage of pneumatic compression devices, and ensuring early mobilization after the surgery.
However, when a clot travels all the way to the lungs, it can lead to a fatal situation called Pulmonary Embolism that may result in chest pain, breathlessness, or even death.  Pulmonary Embolism is a rare complication. A 90-day risk rate for PE with chemoprophylaxis and other measures has been reported as 0.81% which is less than one patient in a pool of hundred. Mortality risk of PE is even smaller.  Another study found the risk-rate of only 0.48% for PE. 
Knee Implant Infection
The clinical signs and symptoms of infected TKR include constant pain, swelling, redness, local warmth or drainage after TKR. Examination findings include tenderness and joint stiffness and limitation of range of motion and painful range of motion of the knee joint that is new or disproportionate to the expected recovery from the surgery.
The cause that leads to failure of knee replacement surgery might include aseptic loosening,prosthesis infections, dislocations, massive bone loss, metal allergy and fractures. After total joint replacement surgery, the total infection rates of the implants are reported to be less than 1%, but patients with rheumatoid arthritis could have a higher infection rate of up to 3.7%.
A bacterial infection of a joint component is a severe complication that is currently difficult to cure with antibiotics. In most cases, the infected joint prosthesis implant has to be removed to cure the infection. It is, therefore, important to improve our understanding of the pathogenesis of prosthesis infections after orthopedic joint replacement.
Knee Surgery Wound Infection
The chance of having a post-surgical wound infection for knee replacements is reported to be about 1% in otherwise healthy patients.  Few other studies indicate a rate of 4% of the otherwise healthy group, and 6-7% for obese class II (35-39.9 KG/m2) and obese class III (> 40 KG/m2).  People who undergo revision surgery (second surgery to replace an implant) have a higher risk of developing such an infection. Habits such as smoking can also lead to a higher rate of both wound and implant infection.
Stiffness or loss of movement
Somepatients may experience stiffness in the knee joint after the surgery. Usually, a stiff knee before surgery is more likely to remain stiff after surgery; however, any knee can lose motion after surgery. It is important to the patient work hard with the physiotherapist after surgery to prevent the knee from getting stiff. 
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After knee replacement, some numbness may be present on the lateral side of the incision for a few months. This numbness does not typically cause any discomfort to the patient. Although in rare cases, nerves of your feet may be injured during the operation. Moreover, these nerves may or may not recover by themselves.
There is a small risk that nerve injuries can happen during/after surgery. However, chances are less, estimated to be less than 1 in a 100. 
Rarely, the arteries and blood vessels around the knee joint may be damaged during operation,and excessive bleeding may occur during or after the operation. In these situations, a vascular surgeon can usually repair the arteries if there any damage is present.
Allergy to metal implants
Rarely, some people may suffer a reaction to the metal used in the artificial knee implants. In people with metal allergies, implant parts trigger a response that produces symptoms that include swelling, a rash, and blisters. If so, make sure to tell your surgeon about the allergy well before surgery. Take the time to discuss the topic with your orthopedic surgeon or medical team if you are unsure.
The North American Skin Patch testing group in 2009 reported the results of testing almost 5000 patients to represent across-section of the population to a wide variety of materials. They found that nickel (Ni) was the most common reactant (21%) with other substances found in orthopedic implants (cobalt 8% and Chrome 8%) were on the rise. Symptoms that associated with metal hypersensitivity included: pain, swelling, skin rash, patient dissatisfaction, and loss of function. 
The femur, patella, or tibia can break during the operation when the surgeon is preparing the bone or implanting the components. Fractures can also occur years after the knee replacement operation from minor trauma. Moreover, these fractures were fixed with metal plates and screws, and sometimes a knee revision surgery was done. 
According to a study published in NCBI, fractures are observed in 0.3% -2.5% (out of 100) of TKA patients mostly within 2 to 4 years after surgery.
After knee replacement surgery some times fixed components may loosen from the bone and change position. Component loosening can occur years after the surgery from wear debris from the plastic liner. The movement of the loose component may cause activity-related pain and require another surgery to revise the components. 
You can help minimize your chances of risks from knee replacement surgery by
- Choosing a surgeon and a hospital that provide high-quality care
- Talking with your doctor long before your knee surgery
- Finding out what you can do to help prevent problems during and after surgery
After having a knee replacement surgery, contact your doctor if
- If you develop redness, warmth, hard or painful areas in your legs in the first few weeks after knee replacement surgery –although this may just be bruising from the surgery, it could mean a blood clot has developed
- If you experience chest pain or breathlessness –although it is very rare, you could have a clot in your lung (pulmonary embolism) which needs urgent treatment.
In summary, if you are well-aware of all possible knee replacement complications, it will not only help you have an honest dialogue about these but can also help you get prepared well for the surgery. In some cases, you may even decide that the risks are greater than the benefit, and decide to skip it all together.
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- Infection post-total knee replacement: current concepts Alijanipour, Pouya; Parvizi, Javad Current Reviews in Musculoskeletal Medicine , Volume 7 (2) – Jun 1, 2014
- Lieberman JR, Geerts WH: Current concepts review: prevention of venous thromboembolism after total hip and knee arthroplasty. J Bone Joint Surg Am 76:1239-1250, 1994
- Khaw FM, Moran IM, Smith SR: The incidence of fatal pulmonary embolism after knee replacement with no prophylactic anticoagulation. J Bone Joint Surg Br 75:940-941, 1993
- Bilateral Total Knee Replacement: Staging and Pulmonary Embolism; Barrett, Jane; Baron, John A.; Losina, Elena; Wright, John; Mahomed, Nizar N.; Journal of Bone and Joint Surgery, Volume 88 (10): 2146 – Oct 1, 2006
- The Incidence of Pulmonary Embolism and Deep Vein Thrombosis After Knee Arthroplasty in Asians Remains Low: A Meta-analysis; Woo-Suk Lee, Kang-Il Kim, Han-Jun Lee, Hee-Soo Kyung, Seung-Suk Seo; Clinical Orthopedics and Related Research, May 2013, Volume 471, Issue 5, pp 1523-1532
- Are Bilateral Total Joint Arthroplasty Patients at a Higher Risk of Developing Pulmonary Embolism Following Total Hip and Knee Surgery? Yeager, Alyssa M.; Ruel, Allison V.; Westrich, Geoffrey H.The Journal of Arthroplasty, Volume 29 (5) – May 1, 2014
- Higher Body Mass Index Leads to Longer Operative Time in Total Knee Arthroplasty; Liabaud, Barthelemy; Patrick, David A.; Geller, Jeffrey A; The Journal of Arthroplasty, Volume 28 (4) – Apr 1, 2013
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